I’m summarising my findings here since the community’s current difficulties installing the latest /e/OS on OnePlus don’t often mention the 7 Pro.
The complete procedure I eventually used to get the latest /e/OS (T (dev)) to work is here (https://community.e.foundation/t/oneplus-8t-cant-shutdown-touchscreen-doesnt-work/50454/26). Some notes about my questions above:
The (LineageOS > guacamole) page continues to list supported models of the 7 Pro, even though the /e/OS page has for some reason dropped them. LineageOS apparently doesn’t list the GM1915 because T-Mobile (and Sprint for other OnePlus models as well) have changed the memory partitioning to discourage installation of Custom ROMs.
An essential means of loading different OxygenOS versions, the Oxygen Updater app, is a standard part of OnePlus tooling and specifically it says you should never try to install a custom ROM on the GM1915 or any carrier-made OnePlus unless you expect to brick the phone (I saw this when running this app on the GM1917).
The one way to universally unbrick a OnePlus is MSMDownloadTool, for which the commonly referenced XDA post doesn’t list the GM1915 as a supported model. I don’t know if that means it wouldn’t work, but I wanted to be sure the installation was reversible… since return for the phone in its original condition could be refunded.
In hindsight, the process that seemed wasteful & wrong when I wrote about it above… using MSMDownloadTool to revert to OOS 11, then unlocking the bootloader & then updating to OxygenOS 12 via the third-party Oxygen Updater app… turned out to be the easiest part of the process. The difficult part has been the S builds of /e/OS not working for the OnePlus platform, as described in other threads. 